Unmanned aerial vehicle (UAV) beam pointing and data rate optimization for high throughput broadband access

ABSTRACT

Systems and methods configured to form and manage different types of beams toward target ground terminals to “optimally” communicate with the terminals. In one set of embodiments, the UAV generates a set of beams to cover cells on the ground, the beams are divided into groups, and the UAV communications system deterministically and sequentially turns a subset of the beams on/off to reduce cross-beam interference and increase system throughput. In another embodiment, in order to increase throughput, the UAV communications system determines the highest data rate on the downlink and uplink that are decodable at the receiver given the received signal to interference plus noise ratio (SINR) while maintaining a low packet error rate. Systems and methods are described to determine the UAV antenna pattern toward different terminals needed for SINR calculation and data rate determination.

PRIORITY

This application claims the benefit of priority to co-owned and U.S.Provisional Patent Application Ser. No. 62/333,088, entitled “UNMANNEDAERIAL VEHICLE (UAV) MULTI-LAYER BEAM POINTING AND SCHEDULING TOWARDGROUND TERMINALS FOR BROADBAND ACCESS”, filed May 6, 2016, which isincorporated herein by reference in its entirety.

COPYRIGHT

A portion of the disclosure of this patent document contains materialthat is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent files or records, but otherwise reserves all copyrightrights whatsoever.

BACKGROUND 1. Technological Field

The present disclosure describes aspects of a system for broadbandinternet access using unmanned aerial vehicles (UAVs) to relay internettraffic among different types of terminals. The present disclosuredescribes systems and methods for “optimally” communicating with groundterminals by manipulating coverage area, transmit power, scheduling,etc.

2. Description of Related Technology

As internet traffic has increased, new technologies are needed todeliver broadband access to homes and enterprises at lower cost and toplaces that are not yet covered. Examples of current broadband deliverysystems include terrestrial wired networks such as DSL (DigitalSubscriber Line) on twisted pair, fiber delivery systems such as FiOS(Fiber Optic Service), and geo-stationary satellite systems. The currentbroadband access systems have a number of short comings. One issue isthat there is a lack of service provided to remote and/or lightlypopulated areas. Geo-stationary satellites do provide service in remoteareas of the developed world such as the United States. However, poorerareas of the world lack adequate satellite capacity.

A notable reason satellite capacity has not been adequately provided inpoorer regions of the world is the relatively high cost of satellitesystems. Due to adverse atmospheric effects in satellite orbits,satellite hardware must be space qualified and is costly. Launchvehicles to put the satellites in orbit are also costly. Moreover, dueto the launch risk and the high cost of satellites, there may besignificant insurance costs for the satellite and the launch. Therefore,broadband satellite systems and services are relatively costly anddifficult to justify, particularly in poorer regions of the world. It isalso costly to deploy terrestrial systems such as fiber or microwavelinks in lightly populated regions. The small density of subscribersdoes not justify the deployment cost.

Hence what are needed are improved methods and apparatus for providingbroadband access to consumers. Ideally such methods and apparatus wouldrely on an inexpensive technology which avoids costs associated withlaunching and maintaining satellites.

SUMMARY

The present disclosure describes, inter alia, systems and methods foroptimally communicating with ground terminals by manipulating coveragearea, transmit power, scheduling, etc.

In a first aspect, an unmanned aerial vehicle (UAV) broadband accesssystem is disclosed. In one exemplary embodiment, the UAV broadbandaccess system includes: an antenna sub-system including at least oneantenna aperture configured to form at least a plurality of beams towarda ground coverage area, where the plurality of beams are subdivided intoa plurality of groups of co-active beams; wherein at least a portion ofthe plurality of groups of co-active beams are co-frequency; a UAV radiosub-system including one or more transmitters and receivers andconfigured to transmit and receive signals to and from a set of groundterminals within the ground coverage area. In one exemplary embodiment,the UAV radio sub-system further includes logic configured to: assign atleast one terminal in at least one beam to transmit during a given timeslot; and measure an uplink signal strength received from the assignedat least one terminal in the time slot on at least one co-activeco-frequency beam.

In one variant, the UAV radio sub-system further includes logicconfigured to: determine an optimal uplink data rate that can be decodedon the uplink based on the measured uplink signal strength and at leastone criteria; and receive uplink data packets based at least in part onthe determined optimal uplink data rate. In one such case, the UAV radiosub-system further includes logic configured to send the measured uplinksignal strength or the determined optimal uplink data rate to theassigned at least one terminal of the set of ground terminals.

In a second variant, the UAV radio sub-system further includes logicconfigured to: schedule downlink transmissions to the assigned at leastone terminal in the at least one beam; and

receive measured downlink signal strength estimates from the assigned atleast one terminal on the at least one co-active co-frequency beam. Inone such case, the UAV radio sub-system further includes logicconfigured to: determine an optimal downlink data rate based at least inpart on the received measured downlink signal quality estimates and atleast one other criteria; and transmit downlink data packets based atleast in part on the determined optimal downlink data rate.

In a third variant, the UAV radio sub-system further includes logicconfigured to: estimate one or more UAV uplink beam gains based at leastin part on one or more of: one or more UAV position coordinates, one ormore UAV orientations, at least one terminal location; determine anoptimal uplink data rate based at least in part of the estimated one ormore UAV uplink beam gains; and receive uplink data packets based atleast in part on the determined optimal uplink data rate. In one suchvariant, the estimated one or more UAV uplink beam gains are performedat a plurality of UAV positions on a cruising orbit.

In a fourth variant, the UAV radio sub-system further includes logicconfigured to: compute an uplink interference power of an interferinguplink of at least one other terminal that transmits during a same timeslot as the assigned at least one terminal; determine an optimal uplinkdata rate based at least in part of the computed uplink interferencepower; and receive uplink data packets based at least in part on thedetermined optimal uplink data rate.

In a fifth variant, the UAV radio sub-system further includes logicconfigured to: compute a total uplink interference based on allterminals that are scheduled to transmit at the same time slot as theassigned at least one terminal; compute an expected signal strength fromthe assigned at least one terminal; compute an expected uplink signalquality of the assigned at least one terminal based on the total uplinkinterference and the expected signal strength from the assigned at leastone terminal; determine an optimal uplink data rate based at least inpart of the computed uplink signal quality at the assigned at least oneterminal; and receive uplink data packets based at least in part on thedetermined optimal uplink data rate.

In a sixth variant, the UAV radio sub-system further includes logicconfigured to: estimate one or more UAV downlink beam gains based on oneor more of: one or more UAV position coordinates, one or more UAVorientations, and at least one terminal location; determine an optimaldownlink data rate based at least in part of the estimated one or moreUAV downlink beam gains; and transmit downlink data packets based atleast in part on the determined optimal downlink data rate.

In a seventh variant, the UAV radio sub-system further includes logicconfigured to: compute an downlink interference power of an interferingdownlink of at least one other terminal that receives during a same timeslot as the assigned at least one terminal; determine an optimaldownlink data rate based at least in part of the computed downlinkinterference power; and transmit downlink data packets based at least inpart on the determined optimal downlink data rate.

In an eighth variant, the UAV radio sub-system further includes logicconfigured to: compute a total downlink interference based on allterminals that are scheduled to receive at the same time slot as theassigned at least one terminal; compute an expected signal strength atthe assigned at least one terminal; compute an expected downlink signalquality of the assigned at least one terminal based on the totaldownlink interference and the expected signal strength at the assignedat least one terminal; determine an optimal downlink data rate based atleast in part of the computed downlink signal quality at the assigned atleast one terminal; and transmit downlink data packets based at least inpart on the determined optimal downlink data rate.

In a second aspect, a method is disclosed. In one embodiment, the methodincludes: forming at least one beam toward a ground coverage area;measuring an uplink signal estimate of received signals on the formed atleast one beam; determining an optimal uplink data rate that can bedecoded on the uplink based on the measured uplink signal estimate andat least one criteria; and receiving uplink data packets on the formedat least one beam based at least in part on the determined optimaluplink data rate.

In one variant, the method further includes: transmitting a referencesignal on at least one other beam; receiving a measured downlink signalestimate of the at least one other beam from at least one terminalwithin the ground coverage area; determining an optimal downlink datarate based at least in part on the received measured downlink signalestimate and at least one other criteria; and transmitting downlink datapackets based at least in part on the determined optimal downlink datarate. In one such variant, the method further includes transmitting themeasured uplink signal estimate to the at least one terminal within theground coverage area.

In a third aspect, a ground terminal apparatus is disclosed. In oneembodiment, the ground terminal apparatus includes: a radio transceiverconfigured to transmit and receive signals to and from the UAV; aprocessor coupled to the radio transceiver. In one exemplary embodiment,the UAV further includes logic configured to cause the ground terminalapparatus to: determine one or more beams that are associated with theUAV; determine one or more time slots that are assigned fortransmission; transmit one or more reference signals according to thedetermined one or more time slots and one or more beams; wherein thetransmitted one or more reference signals is associated with the UAV.

In one variant, the ground terminal apparatus further includes: one ormore instructions which when executed by the processor are configured tocause the ground terminal apparatus to: measure a downlink signalestimate of received signals on a downlink; determine an optimaldownlink data rate that can be decoded on the downlink based on themeasured downlink signal estimate and at least one criteria; and receivedownlink data packets based at least in part on the determined optimaldownlink data rate.

In a second variant, the one or more reference signals are furtherencoded with a UAV beam specific code that identifies the determined oneor more beams from a plurality of beams.

In a third variant, the determined one or more beams are furtherassociated with a group of co-active beams. In some cases, each group ofco-active beams is further associated with one or more frequencies.

In yet another aspect, a communications system configured to assign andmeasure uplink and downlink resources is disclosed. In one embodiment,the communications systems measures uplink and downlink performance atvarious points along an orbit of a UAV. In one such variant, thecommunications systems forms at least one beam in accordance withmeasured performances. Various embodiments thereof may attempt tomaximize or minimize various system parameters, including withoutlimitation, service, power consumption, and/or interference.

In yet another aspect, a communications system configured to form atleast one beam toward a location is disclosed. In one embodiment, thecommunications system is configured to generate at least one beam thatbased on various measured uplink and/or downlink performances. Variousembodiments may additionally be directed to schemes for selectivelymanaging coverage area, transmission and reception scheduling, andfrequency usage.

These and other aspects shall become apparent when considered in lightof the disclosure provided herein.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following figures, where appropriate, similar components areidentified using the same reference label. Multiple instances of thesame component in a figure are distinguished by inserting a dash afterthe reference label and adding a second reference label.

FIG. 1 is a graphical depiction of an exemplary aerial platform basedcommunications system useful in conjunction with various embodimentsdescribed herein.

FIG. 2A is a graphical depiction of exemplary radio equipment of anaerial platform useful in conjunction with various embodiments describedherein.

FIG. 2B is a graphical depiction of exemplary radio equipment of aground terminal useful in conjunction with various embodiments describedherein.

FIG. 3A is a graphical depiction of an exemplary aerialplatform/unmanned aerial vehicle (UAV) at a given altitude and a networkof contiguous beams formed over the coverage area on the ground, inaccordance with at least one embodiment described herein.

FIG. 3B is a graphical depiction of a beam structure formed over thecoverage area on the ground, that includes: a broad beam covering a cellcoverage area and focused beams with smaller coverage areas, inaccordance with at least one embodiment described herein.

FIG. 3C is a graphical depiction of an exemplary aerialplatform/unmanned aerial vehicle (UAV) at a given altitude and a set ofbeams formed over the coverage area on the ground, in accordance with atleast one embodiment described herein.

FIG. 4A is a graphical depiction of data transmission time slots, usefulin conjunction with the various principles described herein.

FIG. 4B is a graphical depiction of exemplary data transmission timeslots and a UAV beam type being used during each time slot according toone embodiment of the present disclosure.

FIG. 4C is a graphical depiction of exemplary data transmission timeslots on two different frequency channels and a corresponding UAV beamtype used during the time slots of each frequency channel according toone embodiment of the present disclosure.

FIG. 5A is a graphical depiction of an exemplary UAV beam forming oncoverage cells on the ground.

FIG. 5B is a graphical depiction of an exemplary UAV beam forming oncoverage cells on the ground.

FIG. 6A is graphical depiction of an exemplary unmanned aerial vehicle(UAV) antenna structure that is configured to form beams toward thecoverage area via mechanical actuators useful in conjunction with thevarious principles described herein.

FIG. 6B is graphical depiction of a multiple type beam system useful inconjunction with the various principles described herein.

FIG. 6C is graphical depiction of a phased array beam forming approachuseful in conjunction with various embodiments of the presentdisclosure.

FIG. 7 is a graphical depiction of a ground terminal with mechanicallysteered antenna composed of multiple antenna apertures useful inconjunction with the various principles described herein.

All Figures © Copyright 2016 Ubiqomm, LLC. All rights reserved.

DETAILED DESCRIPTION

This disclosure describes aspects of a system designed to providebroadband access. As used herein, the terms “unmanned aerial vehicle”(UAV), “aerial platform”, “drone”, refer generally and withoutlimitation to: drones, unmanned aerial vehicle (UAV), balloons, blimps,airships, non-geostationary satellite orbit (NGSO) satellite systemssuch as Low Earth Orbit (LEO) satellites, etc. The aerial platforms mayinclude propulsion systems, fuel systems, and onboard navigational andcontrol systems. In one exemplary embodiment, the aerial platformincludes a fixed wing fuselage in combination with a propeller, etc. Inother embodiments, the aerial platform includes a robocopter, propelledby a rotor. The aerial platform may carry fuel onboard or function usingsolar energy.

FIG. 1 shows one exemplary embodiment of an unmanned aerial vehicle(UAV) 110. As shown, the exemplary UAV 110 has a drone radio sub-system112, a message switch sub-system 116, and at least one drone antennaaperture sub-system 114. UAVs communicate with at least two types ofground terminals: one type of terminals are so-called user GroundTerminals (GT) 120, such as terminals at home or enterprises to provideinternet connectivity to home or enterprise (such as e.g., theInternet); a second type of terminal is referred to as an InternetGateway (GTW) 130 which is connected to the Internet. The embodimentsdescribed hereinafter apply to fixed terminals/devices on the ground, aswell as mobile terminals/devices attached to platforms such as vehicles,boats, ship, airplanes, trucks, etc., and standalone mobile devices(e.g., handheld devices, etc.). The term “device”, as used hereinaftermay broadly encompass any of the aforementioned platforms (e.g., thedrone 110, the GT 120, and/or the GTW 130). During operation, the UAV isconfigured to cruise or patrol an “orbit”, and provide connectivitybetween the ground terminal (GT) 120 and other GT 120 and/or gatewayterminals (GTW) 130. The GTWs 130 may be connected to broader internetnetworks 136, thereby providing the GT 120 internet access and/or accessto other GT or GTW.

FIG. 2A illustrates one exemplary embodiment of an unmanned aerialvehicle (UAV) radio sub-system 112 that includes five (5) sub-systems: areceiver 318 that is configured to demodulate and decode a signalreceived from a drone antenna aperture sub-system 114; a transmitter 316that is configured to modulate data received from a processor 314 andsend the resulting signal through the drone antenna aperture sub-system114; a processor sub-system 314 that is configured to carry outfunctions such as: (i) configuring the receiver 318 and transmitter 316sub-systems, (ii) processing the data received from the receiver 318sub-system, (iii) determining the data to be transmitted through thetransmitter sub-system 316, and (iv) controlling the antenna sub-system114; a non-transitory computer readable memory sub-system 312 that isconfigured to store one or more program code instructions, data, and/orconfigurations, and system parameter information that are accessed bythe processor 314; and a gyroscope/accelerometer/Global PositioningSystem (GPS) sub-system 319 that is configured to determine a positionand orientation of the UAV such as roll/pitch angles. FIG. 2A also showsa scheduler sub-system 314 s which is a part of the processor 314.Scheduler 314 s has two main functions: (i) assigning communicationsbandwidth to different terminals 120 to send/receive data to/frominternet; and (ii) determining which simultaneous beams may be formedduring a communications time unit (e.g., a time slot) toward differentlocations on the ground while maintaining the cross-beam interferenceamong the beams below a certain threshold.

Depending on the altitude of the UAV, each UAV covers an area on theground with a radius of as low as a few 10 s of kilometers (km) and asmuch as 200 km or more. GTs 120 transmit and receive data from theinternet using the UAV 110 as intermediary to the GTW 130. The UAV'sradio sub-system aggregates traffic received from the GTs within thecoverage area of the UAV of a population of GTs (in some implementationsthe UAV may aggregate traffic from as many as all GTs and as few as oneGT) and sends the aggregated data to the internet via one or more of theGTWs. Since, the GTWs handle aggregated data from multiple GTs,practical implementations of the present disclosure may support higherdata rates between the UAV and the GTW, than between the UAV and the GT.Accordingly, in one embodiment the gain of the GTW antenna sub-system ismuch larger than that of the GT, and the GTW transmitter transmits athigher power than the GTs. Those of ordinary skill in the related artswill readily appreciate the wide variety of techniques which may be usedto increase gain, including without limitation, increasingtransmit/receive power, increasing bandwidth, increasing processinggain, increasing coding gain, etc.

Referring back to FIG. 1, the GT 120 has two (2) main sub-systems, aground terminal radio sub-system 122, and a ground terminal antennasub-system 124. As shown in FIG. 2B, the GT radio sub-system 122includes four (4) sub-systems: the receiver 418 that demodulates anddecodes the signal from the drone antenna sub-system; the transmittersub-system 416 that modulates the data and sends the resulting signalthrough the antenna sub-system 124; the processor sub-system 414 thatcarries out functions such as: configuring the receiver 418 andtransmitter 416 sub-systems, processing the data received from thereceiver 418 sub-system, determining the data to be transmitted throughthe transmitter sub-system 416, as well as controlling the antennasub-system 124; and the memory sub-system 412 that contains programcode, configuration data, and system parameters information that areaccessed by the processor 414.

The desired target coverage area on the ground is divided into a numberof cells; one such exemplary division is shown as an arrangement ofthirty seven (37) hexagonal cells in FIG. 3A. The UAV platform formsbeams to cover each cell on the ground in its target coverage area. Asshown, the UAV generates thirty seven (37) beams corresponding to thehexagonal cells; e.g., one (1) “central beam” and three (3) rings ofbeams around the central beam, on the ground. Hexagons are used to showthe ideal coverage of each beam, however in reality, the beams overlapas shown by the dashed circles. In this exemplary example, the availablefrequency bandwidth is divided into three (3) bands (F1, F2 and F3), andthe three (3) frequency bands are assigned to adjacent beams in such away that no two neighboring beams use the same frequency. The threetypes of beams (as represented by a dotted type of circle and two (2)types of dashed circles) may represent beams that use three (3)different frequency channels to minimize inter-beam interference. Theforegoing frequency allocation scheme is described as having a“frequency reuse” of three (3). The three (3) different circle typesindicate beams that use different frequency bands. Moreover, asdescribed in greater detail hereinafter, some embodiments of the UAV andterminal antenna systems (e.g., 114 and 124 in FIG. 1) can be furthermodified to use different polarizations. For example, by using dualcircular polarizations, two data streams can each be distinctly assigneddifferent antenna polarization (e.g., clockwise and counter-clockwise),thereby doubling the system throughput. Those of ordinary skill in therelated arts, given the contents of the present disclosure, will readilyappreciate that other frequency reuse schemes and/or cell divisions maybe interchangeably used with equal success.

Aerial platforms (such as UAVs) cruise/patrol in a three dimensional(3D) space (e.g., latitude, longitude, and altitude). The position ofthe aerial platform/UAV with respect to the terminals on the groundchanges as the aerial platform/UAV moves horizontally and verticallywithin its cruising orbit (e.g., a circle, figure eight, clover leaf,etc.).

The beams depicted in FIG. 3A may be fixed on the ground using a UAV 110antenna system 114 capable of dynamically moving the beams such as tokeep the beam fixed on the ground as the UAV travels in its orbit, orthe beams may be fixed with respect to the UAV 110 in which case thebeams will move on the ground as the UAV travel in its cruising orbit.The beam network of FIG. 3A shows only one beam size (circles) coveringa hexagonal cell on the ground where a large number of ground terminalsare located. Artisans of ordinary skill in the related arts will readilyappreciate that circles are conceptual depictions of the actual beamsthat are formed by the UAV antenna sub-system to cover different cellareas on the ground; actual coverage may vary depending on a variety offactors including but not limited to obstacles, reflections, weatherconditions, UAV positioning (pitch, yaw, roll), etc.

One benefit of using a single beam to cover a large number of terminalsis that the terminals may continuously monitor the beam that covers thearea where the terminal is located, and point the terminals beam towardthe UAV based on signal strength measurements made on signals receivedfrom the UAV. This feature can be used to efficiently send/receive shortburst of data traffic to/from the UAV. However, single beam embodimentsare not always desirable; thus multi-beam embodiments may be used, as isdescribed in greater detail herein.

FIG. 4A is an exemplary depiction of how the available channel bandwidthis divided into time slots, where time slots T1 through T10 are shown.In one such embodiment, there are two (2) types of time slots defined:shared time slots (time slots T1 and T6 in FIG. 4A); and dedicated timeslots (time slots T2 through T5, and T7 through T10). The differenttypes of time slots, shared and dedicated, may also be referred to asshared and dedicated channels in the sequel. Shared time slots, as thename implies, are shared by a number of terminals. Dedicated channelsare assigned to specific terminals one at a time. Generally, “shared”usage allows for flexible resource use based on resource arbitrationand/or access schemes (e.g., code division multiple access,Aloha/slotted Aloha, carrier sense multiple access, etc.) In contrast,“dedicated” usage allocates resources in a fixed manner by a centralizedresource management entity (obviating network overhead for resourcearbitration.)

In some embodiments, the time slot division and designation of FIG. 4Aapplies to both uplink and downlink directions between the UAV and theterminal. In other embodiments, the time slot divisions and/ordesignations between uplink and downlink directions may beasymmetrically allocated. During a shared time slot, the entirebandwidth is shared among the terminals in a given cell coverage areacovered by the broad beam. In some such variants, the available channelfrequency bandwidth may be divided into smaller frequency channels, andeach of the constituent frequency channels may be shared among a certainnumber of terminals. Similarly, during a dedicated time slot theavailable frequency bandwidth may be divided into a number of smallerfrequency channels, and each of the constituent frequency channels maybe assigned to different terminals in a beam's coverage area, or theentire bandwidth may be assigned to one terminal.

Various embodiments of the present disclosure may use fixed resourceallocations or dynamically change resource allocations based on usageconsiderations. For example, when a ground terminal has data to send tothe UAV, it can either send the data on a shared uplink time slot to theUAV, or the ground terminal can use an uplink shared time slot torequest a bandwidth allocation for an uplink dedicated time slot to sendits data. Artisans of ordinary skill in the related arts will readilyappreciate that the overhead needed for a terminal to request bandwidth,and for the UAV to assign the bandwidth to the terminal, is inefficientunless there is a large enough burst of data to be sent. Thus, it may bemore efficient to send data on a shared uplink data channel forsignaling messages between the terminal and the UAV and/or for shortbursts of data packets generated by the terminals or the internet.Moreover, when the terminal is joining the system after power up or hasbeen idle for a time period, the terminal only needs to send relativelyshort signaling messages (which can be efficiently sent on a sharedchannel) to e.g., register with the system, or to inform the UAV radiosub-system 112 that the terminal intends to start a session. But whenthe terminal has a relatively large amount of data to send, it is moreefficient to allocate a certain amount of bandwidth to the terminal on adedicated uplink data channel. Similarly, in the downlink direction, theUAV may aggregate signal messages and short bursts of data destined todifferent terminals in a shared downlink channel in order to improve thebandwidth efficiency of the system; aggregating data for the downlinkchannel avoids/minimizes partially filled physical layer frames, andalso reduces delay. When the UAV has accumulated enough data to send toa terminal, it may then allocate bandwidth to the terminal on adedicated downlink channel. In some cases, the terminal may consider avariety of factors including e.g.: the amount of accumulated data,delivery time requirements for the accumulated data, elapsedaccumulation time, network congestion, etc.

Example Operation—

While the foregoing descriptions provide a generalized framework foroptimizing network coverage via one or more UAVs, various aspects of thepresent disclosure are directed to further improvements for UAV coveragebased on intelligent management of beam interference by manipulatingcoverage area, transmit power, scheduling, etc. More directly, thefollowing discussions provide interference mitigation strategies whichmay be used either alone or in combination to e.g., maximize coveragearea and data throughput/latency, while also minimizing powerconsumption and interference.

Dynamic Multi-Layer Beam Pointing—

In a first aspect of the present disclosure, the UAV dynamicallyswitches between multiple “layers” of coverage area. Broader layers ofcoverage provide larger coverage areas, but may be limited in data.Focused layers of coverage provide smaller coverage areas that cansupport higher data rates e.g., for dedicated use. For example, in oneembodiment, the uplink and downlink shared channels are transmitted on abeam that is wide enough to cover all terminals in a certain definedcell coverage area. However, since dedicated uplink and downlinkchannels only service one terminal at a time (or a number of terminalsthat are in close proximity to one another), the data rate from/to theterminal may be increased by generating a more focused, and thereforehigher gain, beam. Accordingly, in one such variant, the UAV generatesbroad beams that cover the whole cell coverage area when shared channelsare transmitted, and generates focused beams with higher gain to servicethe time dedicated channels; this configuration provides higherthroughput and bandwidth efficiency for dedicated use (i.e., whenresources are dedicated for specific user(s)).

FIG. 3B depicts one such exemplary beam structure that covers a certaincell area composed of a broad beam 520B-1 covering a cell coverage area,and focused beams 520E-1, 520E-2 and 520E-3 toward terminals 120-jduring communications on dedicated channels, where j is an indexidentifying different terminals of the same type in a smaller coveragearea. As shown in FIG. 3B, even within the focused beams there may be anumber of terminals. Thus, in some variants, the UAV may use shared timeslots to load balance the focused beams when it is beneficial tocommunicate with multiple terminals within a focused beam's coveragearea. FIG. 4B depicts an example where some time slots are transmittedon broad beams, shown by ellipses, and some on focused beams, shown bycircles.

FIG. 3C illustrates a UAV broadband access system where the UAV forms aset of non-contiguous beams. In this case, the beams formed by the UAVare fixed on certain locations on the ground for at least the timeduration that data is being transferred to/from terminals in thecorresponding beam locations, such as beams 520B-1, 520B-2 and 520B-3 ofFIG. 3C. The beams may be dynamically moved between locations wherethere are terminals. As shown in FIG. 3C, there are two (2) types ofbeams, broad beams denoted by 520B, and focused beams within each broadbeam denoted by 520F. In the system of FIG. 3C, the beams are fixed withrespect to the ground using antenna techniques with electronic beamsteering capability such as phase array antennas. In otherimplementations, the beams may be fixed with respect to the UAV, ordynamically moved according to some other consideration (e.g., toaugment coverage, etc.)

As discussed above, the focused beam is used to increase the antennagain, and therefore the data rate, when sending/receiving data to/fromone or a small ground of terminals that are in close proximity to oneanother. In order to focus the beam on a small area as shown in FIG. 3B(beams 520E-1, 520E-2, and 520E-3), the UAV determines the positioncoordinates of the specific terminal or the small area. More directly,in order to maximize the Signal to Interference plus Noise Ratio (SINR)seen by the ground terminals and at the UAV, the UAV scheduler (314 s inFIG. 2A) determines the locations to steer focused beams. Variousschemes for location determination may be implemented by those ofordinary skill in the related arts, given the contents of the presentdisclosure. Common examples include e.g., satellite positioning (e.g.,Global Positioning System (GPS), gyroscope/accelerometer tracking,triangulation from known beacons, etc.)

So-called “cross-beam” interference occurs when two or moresimultaneously focused beams use the same frequency band (also referredto as so-called “co-channel” interference). In one exemplary embodiment,the UAV scheduler further monitors cross-beam interference among thebeams, to ensure the interference remains below a certain threshold. Forexample, the scheduler may ensure that the simultaneous co-frequencyfocused beams are far enough apart such that the cross-beam interferenceamong the beams is below the acceptable thresholds. Various metrics maybe used by the scheduler to determine which focused beams may be formedwhile not exceeding the cross-beam interference. For example, one suchscheme maintains a minimum distance between centers of the simultaneousco-frequency beams based on a distance metric. Another scheme would beto compute that cross-interference between the multiple candidatesimultaneous co-frequency beams, using knowledge of the focused beams'antenna patterns; and to schedule those beams whose cross-interference(based on a cross-interference metric) do not exceed acceptablethresholds. In one such variant, when two simultaneous focused beams areclose enough to exceed the cross-beam interference when using the samefrequency band (such as under a frequency reuse of one (1) scheme), thenthe scheduler may assign different frequency bands/channels to thefocused beams.

In the exemplary broad beam network shown in FIG. 3A, a frequency reusefactor of three (3) is used for the broad beams in order so as tominimize cross-beam interference. However, in some embodiments, duringthe time slots when focused beams are formed, the scheduler may (asdescribed above) schedule focused beams that are on the same frequencyband (i.e., with a frequency reuse of one (1)) for different time slots,thereby making more efficient use of the spectrum.

Sequential Beam Pointing—

In the foregoing dynamic multi-layer beam pointing embodiments, thefocused beams are dynamically allocated to terminals while ensuring thatthe SINR seen by any terminal remains above a certain threshold. Tothese ends, the scheduler assigned beams to different terminals tomaximize (or minimize) certain performance metrics, such as totalthroughput from all terminals, or the SINR each terminals receives, etc.In a second aspect of the present disclosure, the UAV minimizescross-beam interference by sequentially using certain predefinedbeams/cells in such a way as to minimize interference among beams; forexample, to maximize SINR seen by a specific terminal or the UAV radiosub-systems in order to maximize system throughput. In one such variant,the entire coverage area is divided into a number of cells covered bybeams, and the scheduler sequentially turns on or off various cellsthereof.

In at least one exemplary implementation, the sequence of activatingcells is based on a deterministic scheme. Deterministic schemes allowfor simpler coordination between the UAV and the ground terminals,thereby reducing unnecessary power consumption (i.e., ground terminalsdo not need to turn on when there is no active coverage). Commonexamples of a deterministic schemes include e.g., pseudo-randomassignments, fixed sequence assignments, etc.

FIG. 5A shows an exemplary diagram of how an exemplary coverage area onthe ground may be divided into different cells covered by beams. Asshown, the area is divided into conceptual hexagonal cells which arecovered by beams formed by the UAV antenna sub-system. The hexagons aregrouped into groups of three (3) as represented by different hash markedfills. Each group is serviced under the same frequency channel. Each ofthe groups corresponds to a different frequency. In other words, a firstgroup may use a frequency channel F1, a second group uses frequencychannel F2, and a third group uses frequency channel F3.

In the system depicted by FIG. 5A, there are three (3) sets of co-activebeams 1, 2 and 3. In one such variant, only one of the co-active set ofbeams may be turned on at a time, while the other sets of co-activebeams are off. For example, the UAV radio sub-system first transmits onco-active beams numbered 1, while keeping co-active beams numbered 2 and3 off. Once the terminals in the cells numbered 1 are served, then theUAV radio sub-system turns co-active beams numbered 1 off, and powers onco-active beams numbered 2 to transmit/receive data to/from terminals inthe coverage area of the co-active beams numbered 2. Next, co-activebeams numbered 3 are next turned on, while keeping co-active beamsnumbered 1 and 2 off. The coverage area arrangement and beam schedulingscheme described above reduces co-frequency interference among beams byspatially separating use; more directly, the same frequency is not usedin the nearest beams/cells. Deterministic sequential beam pointingallows for high gain spot beams to increase the overall system's gainand throughput. Artisans of ordinary skill in the related arts willreadily appreciate that the foregoing discussion is purely illustrative;a greater or fewer number of groups and/or co-active beams may besubstituted with equivalent success, given the contents of the presentdisclosure. Moreover, while the foregoing example is illustrated withonly one active set of beams at a time, those of ordinary skill in therelated arts will readily appreciate that multiple sets of co-activebeams may be on at the same time, so long as the cross-beam interferenceremains within acceptable levels.

FIG. 5B illustrates an alternative variant of the UAV beam formingnetwork, where beams covering all cells use the same frequency channel(as distinguished from the different frequency channels in FIG. 5A). Asshown, the network of FIG. 5B also divides the coverage area into groupsof three (3) co-active cells (also shown numbered 1, 2 and 3). As in thesystem described in conjunction with FIG. 5A, the UAV forms beams onlytoward one set of co-active cells at a time. In the system of FIG. 5B,however, unlike the frequency reuse scheme of three (3) in FIG. 5A, theco-active beams of FIG. 5B use the same frequency channel (i.e.,frequency reuse of one (1)). A frequency reuse of one (1) makes betteruse of the available spectrum as it assigns more spectrum to each beamwithout dividing the available spectrum into smaller channels (e.g., aset of three (3) or more channels). However, a frequency reuse of one(1) can introduce more cross-beam interference if the system is notdesigned to adequately isolate co-active beams. Consequently, in thesystem of FIG. 5B, the cross-beam interference is managed by stipulatingthat the UAV will be transmitting on only a subset of the co-active beamsets at a time. For example, the UAV only transmits on one (1) cell ofthe co-active cells numbered 1; the other neighboring co-active cellsnumbered 1 do not receive transmissions thereby reducing cross-beaminterference. Notably, the ground receivers located within the co-activecells numbered 1 may or may not be active for the transmit time.

In certain exemplary embodiments, the neighboring beams that belong todifferent co-active beam groups (e.g., 1, 2 or 3) may be formed by thesame antenna aperture. In some such variants, only one of the three (3)neighboring beams are used, the same antenna aperture can move the beamfrom one cell to another in the three (3) neighboring cells withdifferent co-active cell numbers. In another embodiment, the sameantenna aperture may form multiple beams, each beam covering a differentco-active cell. Artisans of ordinary skill in the related arts willreadily appreciate that increasing/decreasing the number of beams isdirectly proportional to processing gain and power consumption; thus,some implementations may dynamically enable or disable the number ofsimultaneous co-active beams, depending on e.g., the UAV's own internalconsiderations and capabilities, the network's requirements, userrequirements, etc.

Artisans of ordinary skill in the related arts will readily appreciatethat deterministic schemes are inflexible, and thus can be inefficientwhere the network traffic is unpredictable and/or where network trafficis prone to spiking. Accordingly, alternative embodiments may usenon-deterministic schemes where the UAV determines which cells should beserviced based on e.g., historic and/or known traffic requirements, loadbalancing, total outage time (so as to ensure that certain cells receivea minimum level of coverage, even where higher priority cells arepresent), network priority information, etc. Since non-deterministicsequencing is unpredictable, the ground terminal may need to regularlycheck for coverage status; thus, in some variants, the UAV mayadditionally provide beacon type information, or possibly otherout-of-band information, that allows the ground terminals to minimizepower consumption when not being serviced.

In some variants of the systems described above in conjunction withFIGS. 5A and 5B, the UAV beam may cover the whole defined conceptualhexagonal cell, and therefore the same beam provides coverage for sharedand dedicated channels. In other words, in the UAV beam network designembodiments described by FIGS. 5A and 5B, the same beam may allowoperation of either and/or both shared and dedicated channels over thecell's coverage area during the time a given beam is on and iscommunicating with the terminals in its cell coverage area.

Beam-Specific Power Control—

While the foregoing examples illustrate intelligent management ofcoverage area to optimize network efficiency, other aspects of serviceprovision can be adjusted to further improve operation. As a briefaside, network traffic is generally unpredictable and can experiencespikes or bursts of activity. In order to maximize network uptime,network infrastructure is generally capable of supporting excesscapacity, but only enables those resources when necessary. Accordingly,within the context of the present disclosure, so-called “low traffic”cells have ground terminals that have little or no data to send/receive.In some variants, a cell that has low priority (and/or best effortdelivery) traffic may also be considered a low traffic cell.

During normal operation, some cells may have low traffic periods. Forexample, consider the systems where a co-active group of cells includesat least one or more low traffic cells. If power is equally allocated toeach cell of the co-active group of cells then the powers allocated bythe UAV to the beams associated with the low traffic cells are more thanadequate to support their low data rates; in contrast, cells which havespiking data rates (“high traffic” cells) may not have enough gain orprocessing capability to fully support those customers. Thus, in orderto make better use of the available UAV power, the UAV radio sub-system(112 in FIG. 1) can allocate an uneven amount of power for each of itsspiking cells. The power from the low traffic cells can be redirected toboost the power to high traffic cells. In other words, the UAV radiosub-system uses power control to adjust the power allocated to differentUAV co-active beams so as to provide more power to the beams whose cellshave more traffic. Various embodiments of beam-specific power controlmay allocate the boost power for a variety of performance enhancingtechniques. For example, the boost power may be used to increase atransmit power and/or a receive gain of the transceiver.

The beam-specific power control as described above may be further usedto enable higher throughput to beams whose cells have more traffic, andadjust the UAV throughput to traffic conditions in the coverage area.For example, in the UAV beam network design described in conjunctionwith FIGS. 3A and 3B, the UAV forms beams on a contiguous set of cellson the ground that cover a certain coverage area. In one exemplaryvariant, the UAV radio sub-system may allocate less power to beams thatcover low traffic cells and more power to beams that cover the highertraffic cells. Similarly, in the UAV beam network design described inconjunction with FIGS. 5A and 5B, the UAV only transmits to a subset ofthe cells at any particular time; consequently, in one exemplaryvariant, the UAV can utilize much higher power amplification schemes tomaximize data throughput for the co-active cells. Artisans of ordinaryskill in the related arts will readily appreciate that beam-specificpower control is widely applicable to many other different networkconfigurations, given the contents of the present disclosure.

While the foregoing discussion has been presented primarily in relationto power, the various principles described herein may be readily appliedto virtually any limited resource. Common examples of limited resourcesinclude without limitation: power, time, frequency, spreading codes,components (e.g., antennas, processors, etc.), memory, thermaldissipation, etc. For example, a low traffic cell may be allocated fewertime slots or frequency bands during coverage, the excess time slots orfrequency bands being allocated to high traffic cells. Various othersubstitutions may be made with equal success, given the contents of thepresent disclosure.

Link Layer Data Rate Determination—

For network configurations that only service portions of the networkcoverage area (see e.g., Sequential Beam Pointing discussed supra),services that are not in the co-active cells must be postponed fortransmission/reception. One metric for measuring such performance is aso-called network data transfer efficiency that is characterized by theratio of the data that was transferred as a portion of the total datathat was ready for transfer. Another aspect of the present disclosuredescribes systems and methods for optimizing network data transferefficiency. Common examples of data rate measurements may includewithout limitation: bit error rate (BER), packet error rate (PER), blockerror rate (BLER), total data throughput, peak data throughput, averagedata latency, peak data latency, etc.

In one exemplary embodiment, the network data transfer efficiency can beevaluated based on Link Layer Data Rate Determination (LDRD) which isthe amount of data that is transferred at the so-called Link Layer.Specifically, the UAV performs LDRD in order to monitor and maximize theoverall system throughput. During normal operation, the UAV and terminalradio sub-systems determine the highest data rate that the UAV and theground terminal can transact data subject to certain pre-definedcriteria. For example, the highest acceptable data rate may bedetermined to be the data rate that does not exceed a threshold packeterror rate (PER) at the ground terminal and/or UAV receivers; in suchcases, the acceptable PER is determined based on a measured signal tointerference plus noise ratio (SINR) at the terminal and UAV receivers.Other methods for determining the highest acceptable data rate may bebased on other error measurements (e.g., BER, BLER, etc.) and/or otherchannel measurements (e.g., received signal strength indication (RSSI),carrier to noise ratio (CNR), etc.) Still other methods may considerother factors and/or weight different types of data differently based one.g., data requirements (e.g., throughput, latency, etc.), quality ofservice (QoS), customer requirements, etc.

There are two (2) distinct approaches for performing LDRD: (i) so-called“closed loop” and (ii) so-called “open loop” schemes.

In closed loop variants, the UAV and ground terminals communicatequality measurements via a feedback loop between the UAV and terminalradio sub-systems. In other words, the UAV and terminal receivers eachmeasure the SINR received on their respective uplinks and downlinksrespectively, and report the measured SINR to each other. Based on thereported SINR, the terminal and UAV radio sub-systems can transmit dataat the highest rate that is acceptably decodable for the given SINRvalues. In some cases, the acceptable data rate is pre-determined (viae.g., empirical observations of the manufacturer, network simulations,etc.) In other cases, the acceptable data rate is dynamically determinedbased on e.g., historic analysis, predicted conditions, data usage, etc.Alternatively, in open loop variants, the UAV and ground terminals donot exchange quality measurements i.e., there is no feedback loopbetween the UAV and terminal radio sub-system.

As previously alluded to, the primary source of interference that can becontrolled, and adversely affects the measured SINR is cross-beaminterference. Cross-beam interference can be introduced from neighboringco-active and co-frequency beams, interference received from otherantenna polarizations in dual polarization antenna operation, etc. Othersources of interference may be compensated for (by increasing gain,coding complexity), or avoided (e.g., by changing transmission timeintervals, frequency bands, spreading codes, etc.), but the UAV only hasdirect control over which beams are active, and thus how much cross-beaminterference is present. Accordingly, in some variants, the SINR may befurther partitioned into the amount of interference due to cross-beaminterference, and the amount of interference which is attributable tothe radio channel.

In order for the UAV and terminal receivers to measure the receivedSINR, the terminal and UAV transmitters each send a signal (such as 222and 212 in FIG. 1) so that the respective UAV and terminal receivers canmeasure the received SINR on the uplink and downlink channel. While theforegoing example is presented in terms of transacted messages (e.g.,so-called “live” data), other schemes may be substituted with equivalentsuccess given the contents of the present disclosure. For example, insome cases, the UAV and/or terminals could transmit so-called “pilotchannels”, “beacons”, “training sequences”, etc. as are commonly foundwithin the related arts. For such embodiments, a receiver that canreadily identify the portion of SINR directly attributable to cross-beaminterference may provide such identification information for improvedinterference mitigation and/or network utilization analysis. Forinstance, a neighboring beam that introduces cross-beam interferencecould be identified based on its beacon signature, and disabled in theevent that the neighbor's traffic is of a lower priority,

While closed-loop feedback allows for accurate interference mitigationof short distances for UAVs that are located at a sufficiently highaltitude, round trip propagation (RTP) may introduce significantcomplications in a closed loop feedback system. Consider a UAV that islocated fifty (50) kilometers (km) from a ground terminal; in order tomeasure SINR, the terminal must: transmit a signal to the UAV, whereuponthe UAV receiver measures the received SINR and responsively sends themeasured SINR back to the terminal. The total round trip delay isapproximately 333.33 micro-seconds (ms). Since neither the UAV nor theterminal is aware of the channel conditions until they receive the SINRmeasurements, the UAV and terminal must use lower than optimal datarates to avoid high packet error rates. Once each end of the linkreceives the reported SINR measured at the other end, the link canswitch to an optimal rate (e.g., that sustains the acceptable PER, etc.)for the current data transmissions based on the measured SINRs. However,under the closed loop scheme, each SINR update lags the currentconditions by 333.33 ms. For high data rates, the measurement lag can besignificant; for example, when the data links operate at very high datarates (e.g., one (1) gigabit per second (Gbps) or higher), the largeround trip delay can result in a loss of throughput due to less thanoptimal data rates. More directly, since the actual SINR of the channelmay have changed since the reported SINR, the link may be using anunderweight error coding scheme (resulting in larger than anticipatederror rates), or an overweight error coding scheme (resulting in lessdata throughput).

Open loop schemes are less susceptible to time lag issues when comparedto closed loop schemes. However, open loop schemes may be more difficultto accurately coordinate between the UAV and the ground terminals, asthere is no explicit feedback mechanism. Moreover, since the downlink(UAV to ground terminals) may have different considerations than theuplink (ground terminals to the UAV), various embodiments of open loopoperation may require different management techniques which are adaptedto reflect the asymmetric relationship.

Referring now to downlink open loop LDRD, in one exemplary embodiment,the UAV radio sub-system estimates a received signal quality metric(such as SINR) at the ground terminal based on e.g., knowledge of UAVtransmit power, estimated propagation loss from the UAV to the terminal,and the antenna gains from UAV beams toward the ground terminal, etc. Inone such variant, to simplify the description of the embodiment thedescription ignores negligible sources of interference e.g., fromcross-polarization terms (generally, cross-polarization terms aresignificantly smaller than the interference from the main polarizationand can be estimated similar to the cross-beam interference estimationembodiments).

As previously noted, cross-beam interference is a function of the numberand intensity of co-active co-frequency beams. Thus, during datatransmissions between the UAV and a given terminal, the cross-beamco-frequency interference may change whenever co-active co-frequencybeams stop or start transmitting. As the interference component (I) ofSINR changes, the maximum decodable data rate on the corresponding linkwill also change; this may be reflected by changes in acceptable packeterror rate (PER) thresholds. In the open loop LDRD mechanism, the UAVradio sub-system has knowledge of the transmissions to each groundterminal during each time slot, and can therefore estimate the SINR onthe downlink seen by the terminals during each slot. Based on theseestimates, the UAV can change the data rate during each time slot toadjust for changes in the interference conditions.

Consider the following analysis of interference experienced by anexemplary downlink transmission. The UAV serving beam is the UAV beamthat a given terminal is associated with; the UAV serving beamsends/receives data to/from the terminal. Interfering UAV beams use thesame frequency as the UAV serving beam (i.e., co-frequency beams) andare active within the same time interval (i.e., co-active). Aspreviously noted, cross-polarized antenna beams can be ignored tosimplify the description since their effects are negligible. However,the following description may be generalized to also account forcross-polarization interference.

The downlink interference power from co-active co-frequency andco-polarization beams seen by the terminals is given by:DI _(l)=Σ_(j=1) ^(N) DI _(lj)  (1)Where:

DI_(l) is the total co-frequency and co-polarization interference fromall N co-active co-frequency and co-polarized beams at terminal l; and

DI_(lj) is the co-frequency co-polarization interference power receivedfrom the UAV interfering beam j at terminal l during the co-activeinterval.

DI_(lj) is given by:DI _(lj) =A _(l) UP _(j) TG _(l) UG _(Dlj)  (2)Where:

A_(l) is propagation loss from the UAV to terminal l;

UP_(j) is transmit power from the UAV interfering beam j;

UG_(Dlj) is the antenna gain from beam j of UAV toward terminal l in thedownlink direction; and

TG_(l) is the antenna gain from terminal l toward the UAV.

Since the UAV radio sub-system controls and manages the co-activetransmission time slots and/or frames and transmission powers, the UAVradio sub-system can estimate the effects of co-active co-frequencyco-polarization beams on its serviced ground terminals in accordancewith equations (1) and (2) (discussed supra). More directly, the UAVradio sub-system can estimate the interference a given terminal l willreceive from co-active co-frequency beams. Sophisticated variants mayfurther consider both the terminals' position coordinates, and the UAVposition coordinates (and roll, pitch, and yaw) based on the GPS andgyroscope/accelerometer located as part of the UAV radio sub-system. Insuch variants, the UAV radio sub-system can take into account the UAV'sposition and orientation when determining the amount of UAV beam gainstoward the terminal and also when computing the propagation path lossfrom the UAV to the terminals.

Once, the downlink interference power from co-active co-frequency andco-polarization beams has been calculated, the UAV sub-system canestimate the SINR. SINR is given by:

$\begin{matrix}{{SINR} = \frac{S_{l}}{{DI}_{l}}} & (3)\end{matrix}$Where S_(l) is the desired signal power received at terminal l from UAVserving beam that provides coverage to terminal l. S_(l) is given by:S _(l) =A _(l) UP _(l) TG _(l) UG _(Dl)  (4)Where UG_(Dl) is the UAV serving beam gain toward terminal l in thedownlink direction.

After the UAV radio sub-system has estimated the downlink SINR receivedat the terminal based on equation (3) (provided supra) during a givendownlink time slot or frame, the UAV sub-system can select theappropriate data rate to transmit data packets to the terminal based onthe acceptable packet error rates (PER) for the corresponding SINR. Moredirectly, the open loop downlink LDRD estimates the downlink SINR andthe corresponding achievable data rate based on information that ispresent at the time of transmission, and can be calculated without anydelay (i.e., does not require feedback input).

Unlike the UAV, ground terminals do not have the same control over thecommunication medium. Specifically, the ground terminals will need toconsider both shared medium access and dedicated medium access. Considerfirst the uplink dedicated channels, where the UAV radio sub-systemprocessor has assigned certain time slots on the uplink to each groundterminal and sent the channel assignments in a downlink assignmentmessage. Since the UAV radio sub-system processor has generated thechannel assignment for each terminal in different cells/beams during agiven uplink dedicated channel, the UAV radio sub-system processor canestimate all uplink dedicated channel assignments to the differentterminals for the time slot, and thus also estimate the SINR that theUAV will see from each terminal during that time slot.

The uplink interference power from co-active co-frequency andco-polarization terminals is given by:UI _(l)=Σ_(j=1) ^(M) UI _(lj),  (5)Where:

UI_(l) is the total co-frequency co-polarization interference from all Mtransmitting interfering terminals as seen on the uplink by the UAVreceiver when decoding packets of terminal l; and

UI_(lj) is the co-frequency co-polarization interference power receivedfrom interfering terminal j as seen on the uplink by the UAV whendecoding packets of terminal l.

UI_(lj) is given byUI _(lj) =A _(j) TP _(j) TG _(j) UG _(Ulj),  (6)Where:

A_(j) is propagation loss from the UAV to the interfering terminal j;

TP_(j) is transmit power from interfering terminal j;

UG_(Ulj) is the antenna gain from the UAV beam that serves terminal ltoward interfering terminal j in the uplink direction; and

TG_(j) is the antenna gain from interfering terminal j toward the UAV.

The uplink SINR seem at the UAV from terminal l is then given by

$\begin{matrix}{{{SINR} = \frac{S_{l}}{{UI}_{l}}},} & (7)\end{matrix}$Where:

S_(l) is the desired signal power received from terminal l at the UAV,and is given by:S _(l) =A _(l) TP _(l) TG _(l) UG _(l)  (8)Where:

TP_(l) is the terminal l transmit power, TG_(l) is the terminal lantenna gain toward the UAV, and UG_(Ul) is the gain of the UAV beamthat serves terminal l toward terminal l in the uplink direction.

As previously noted, since the UAV radio sub-system processor hasassigned the dedicated channel assignments for the uplink to differentterminals, the UAV radio sub-system can predict which terminals will betransmitting during a given uplink dedicated time slot or frame.Therefore, the UAV radio sub-system can estimate the uplink SINR for allterminals that are scheduled to transmit during a time slot based onequations (5) through (7) described above. Based on the estimatedreceived uplink SINR, the UAV radio sub-system processor determines thedata rate that each terminal should use when transmitting data to theUAV during the assigned uplink time slot. The UAV radio sub-system sendsthe corresponding data rate indices to the terminals in the channelassignment message that contains the terminals' uplink dedicated channelassignments.

As will be appreciated by artisans of ordinary skill in the relatedarts, the UAV beams' gain toward the terminals in the UAV coverage areais a function of the UAV position and orientation (roll, pitch and yaw),as well as the position of the ground terminals. Consequently, in somesophisticated embodiments, the UAV radio sub-system compensates for theUAV position and orientation (roll, pitch, and yaw) when computing theUAV beams' gains toward the terminals used in the interference and SINRestimation equations (1) through (7) described above. In otherembodiments, the UAV radio sub-system compensates for the UAV positionand orientation when steering the UAV beams' gains toward the terminals(electrically and/or mechanically).

Once a data rate is chosen on the downlink to each terminal, then powercontrol may be used to minimize the total power transmitted by the UAVon all beams while ensuring that the required SINR to achieve the chosendata rate is achieved on the downlink to each terminal. Variousembodiments of the UAV radio sub-system may implement any number ofpower optimization schemes; for example, one such scheme minimizes thetotal UAV transmit power. In another example, the UAV radio sub-systemmay minimize the total UAV transmit power over all beams whileconstraining the transmit power on certain beams to specific values.Similarly, the UAV may optimize the transmit power from each terminalaccording to a number of power optimization criteria subject toachieving the required SINR on each uplink based on the chosen data rateon each uplink; under such variants, the UAV sends the computed transmitpowers to each terminal for uplink transmission.

Antenna Pattern Measurement—

Simple embodiments of the present disclosure may operate under theassumption that the antenna pattern is uniform throughout the cellcoverage area; however, more sophisticated embodiments may optimizeperformance depending on antenna pattern measurements. For example, indiscussions of open loop LDRD embodiments supra, the identifiedequations for the UAV radio sub-system either empirically determine,estimate, or assume, the UAV antenna beams' gains toward differentterminal locations as a function of the terminal location as well as theUAV's position and orientation. Accordingly, various aspects of thepresent disclosure are directed to systems and methods for computingand/or estimating the UAV beams' gain toward different terminallocations as a function of the UAV position and orientation.

In one exemplary embodiment, the UAV antenna beams' patterns aremeasured a priori in an outdoor range with respect to different UAVantenna orientations and stored in a table. During subsequent in-serviceoperation, the values in the table can be used by the UAV radiosub-system in the interference and SINR calculations (such as thoseidentified in the open loop LDRD scheme described above). Theaforementioned a priori test measurements are made under typicaloperating conditions (e.g., the UAV antenna structure is manipulated ina manner similar to how it would move during the UAV's normal travel inits cruising orbit, including changes in the orientation of the UAV).Ideally, measurements are made in a realistic scenario that is set up toprovide far field beam patterns as similar as possible to the conditionsin the actual UAV flight.

In another embodiment, the UAV antenna beams' gains toward the differentterminal locations on the ground as a function of UAV's position andorientation may be empirically measured as the UAV cruises in its orbitduring the actual operation of the UAV at altitude. As the UAV cruisesin its orbit, the UAV radio sub-system transmits a known referencesignal that is detected by the terminals on the ground. The terminalssearch for reference signals transmitted from UAV's co-frequency beams,and measure the interference. In the exemplary open loop LDRD variant,the UAV may perform interference measurements based on equation (2),supra. For example, the aforementioned equation (2) can be used toderive the combined UAV beam and terminal antenna beam gains(TG_(l)UG_(Dlj)) based on the path loss from the UAV to the terminal(A_(l)), the receiver integrator processing gain PG, and the interferingco-active co-frequency UAV beam transmit power (UP_(l)) (thesequantities are locally available to the UAV).

With regard to the ground terminal, the ground terminal can computeUG_(Dlj) based on the UAV beam j gain toward terminal l, because theterminal peak antenna gain toward the UAV can be estimated to reasonablecertainty, within the antenna pointing error. Therefore, the differentterminals in the UAV coverage area may make measurements on referencesignals sent by the UAV's co-frequency beams and compute the UAV beams'gains toward the terminal, i.e. the downlink UAV beams' gains UG_(Dlj).Once the ground terminal has calculated UG_(Dlj), the terminals send themeasured UAV beam gains to the UAV. Similarly, the UAV radio sub-systemmeasures the uplink UAV beam gains UG_(Ulj) (see e.g., from equation (6)exemplary open loop LDRD variant), where the terms UI_(lj) can bedirectly measured by the UAV radio based on detecting reference signalstransmitted by the terminals. In one such variant, the reference signalstransmitted by terminals in different UAV beams are further encoded by aUAV beam specific code so that the UAV radio sub-system can determinewhich co-frequency beam a given terminal is communicating.

The empirically gathered uplink and downlink performance data can bestored for future use, and/or uploaded for offline analysis to furtheroptimize performance. In some cases, analysis of multiple sets ofempirical data collected from multiple UAVs may be used to furtherinform network management/modeling and improve operation of an entirefleet of UAVs, etc. To these ends, in one exemplary embodiment, the UAVradio sub-system processor stores: (i) the downlink UAV beam gainsUG_(Dlj) and UG_(Dl) toward the terminal received from the terminals,(ii) the uplink UAV beam gains UG_(Ulj) and UG_(Dl) measured by the UAVradio sub-system, and (iii) the UAV's position and orientation at thetime of measurements, within a data structure. The contents of the datastructure can also be used for in-service operations (e.g., the UAVdownlink and uplink antenna measurements can be used in computing thedownlink and uplink SINR during the downlink and uplink time slotassignment in the open loop LDRD process, etc).

In one exemplary embodiment, the terminals and the UAV make UAV beammeasurements as described above during at least one full pass of the UAVaround its cruising orbit so as to create a table of UAV beam gainstoward each terminal for all UAV positions and orientations. In somevariants, the measurements are repeated periodically as the UAV cruisingorbit and orientation limits may change with time due to externalconditions such as weather conditions (e.g., wind, rain, humidity, etc.)Periodic measurements of UAV antenna beams' gain toward the terminalsensures that the table of UAV beam pattern versus UAV position andorientation has current patterns and information for most (if not all)UAV positions and orientations during operation.

Referring back to equations (2) and (6) of the open loop LDRD embodimentdescribed supra, the interference terms (DI_(lj) and UI_(lj)) arelargely attributed to the co-frequency beams whose undesired sidelobesare parasitically received by the terminal; in most practicalapplications, these interference terms are weak signals. Accordingly, inone exemplary embodiment, the UAV's reference signal is specificallydesigned so that ground terminals can detect the reference signal andcompute the received strength based on the weak signal strength of thesidelobes of the UAV co-frequency beams. In one exemplary variant, thereference signal is repeated over a long time interval to ensuredetection when the signal is weak. In other variants, the referencesignal may be spread over a long spreading code (thereby allowing forrecovery even under high noise environments). In order to recover thespread reference signal, the terminal receiver integrates the knownreference signal over a long enough time period to achieve enoughprocessing gain so that the terminal may even detect weak signals. Insome such variants, the reference signals sent from different UAV beamsare encoded using a beam dependent code so that the terminals may searchfor the reference signals from different beams, and use the interferenceterms associated with different co-frequency beams in estimating the UAVbeam gains (UG_(lj)) toward terminal as described above. Various othermethods for reference signal delivery will be readily appreciated bythose of ordinary skill in the related arts.

The embodiments in this disclosure were described in the context ofUAVs. However, the embodiments also apply to Low Earth Orbit (LEO)satellites, and NGSO satellites systems in general. Specifically, theembodiments regarding the beam network formed in the coverage area ofthe UAV may also be used to form beams for LEO satellites in theircoverage area to maximize the LEO satellite throughput. The embodimentsthat describe LDRD also apply to LEO satellite systems to find thehighest data rate the satellite beams and the terminals on the groundmay send data to maximize system throughput.

Terminal Position Location Determination—

As mentioned above, the position coordinates of the terminals can beused by the UAV in order for the UAV to point a focused beam on theterminal. For example, in one embodiment, the position coordinates ofthe terminal may be measured (e.g., using a GPS device at the locationof the terminal) and sent to the UAV using the link between the UAV andthe terminal on a broad beam. In another embodiment of this disclosure,the UAV may estimate the position coordinates of the terminal usingmultiple range measurements. As mentioned above, the UAV travels on acruising orbit, but the terminal's position is relatively fixed. Theterminal and UAV may make range measurements at multiple locations onthe cruising orbit of the UAV and trilaterate and/or triangulate themeasured range values to estimate the location of the terminal. In onesuch variant, the UAV has knowledge of its position coordinatesP_(c)=(x,y,z) using a GPS device installed on the UAV. The UAV 110 andterminal 120 (FIG. 1) may make Round Trip Delay (RTD) measurement usingthe messages 212 and 222 between the UAV 110 and the terminal 120. TheUAV 110 radio sub-system 112 records the time the message 212 is sent tothe terminal 120. The terminal 120 sends back message 222 to the UAV,message 222 including information such as the time the message 222 istransmitted, to allow the UAV radio sub-system 112 to estimate theRTD(j) between the UAV and the terminal, where j is an integeridentifying the j-th UAV position on the orbit. The RTD(j) measurementis then converted to distance, d(j), using the speed of light andcompensating for UAV movement, etc. As the UAV travels around its orbit,the RTD(j) and the corresponding distance between the UAV and theterminal, d(j), is measured from at least three (3) positions. Then, thedistances between the UAV and the terminal d(j), and the correspondingUAV position coordinates P_(c)(j), are used by the UAV or the terminalin the trilateration algorithms (or triangulation algorithms) toestimate the terminal's position coordinates.

Hand-Off of Terminal Between UAV Beams—

For embodiments of FIG. 3 which have beams that are fixed with respectto the UAV, the beams will move on the ground relative to the terminals,and the terminals must hand-off from one beam to another as the beamsmove. The beam on which the terminal is communicating with the UAV isreferred to as the current beam. The beams that are adjacent to thecurrent beam are referred to as the neighbor beams. The UAV 110 and theterminals 120 measures signal metrics such as Signal to Interferenceplus Noise Ratio (SINR) on signals received on the current and neighborbroad beams, such as 212 shown in FIG. 1. The UAV radio sub-system 112compares the signal quality, such as SINR, of the current beam to thatof the neighbor beams, and if the signal quality of a neighbor beam iswithin a certain threshold of the current beam then the terminal caninitiate hand-off. The beam with strongest signal quality from among theneighbor beam will be chosen as the beam to which to hand-off (referredto as the candidate beam) and the terminal may request that the UAVhand-off the terminal to the candidate beam.

As previously described in FIGS. 5A and 5B, certain embodiments may use“fixed” beams that are fixed with respect to the UAV, the beams willmove on the ground relative to the terminals, and the terminals canhand-off from one beam to another as the UAV beams move. Accordingly, inone exemplary embodiment, the terminal radio sub-system (122 in FIG. 1)makes signal quality measurements, such as SINR, on the referencesignals transmitted from the UAV serving beam, as well as on signals ofthe non-serving beams neighboring the serving beam (when the neighboringbeams are active). The terminal's radio sub-system processor comparesthe signal quality metric measured on the serving beam and thenon-serving neighboring beams. When the signal quality of a neighboringbeam meets certain selection criteria (e.g., greater than the servingbeam, within an acceptable threshold power of the serving beam, hasincreasing signal power relative to the serving beam, etc.), theterminal radio sub-system will attempt to initiate hand-off to this“candidate hand-off beam”. In one such variant, the terminal radiosub-system sends a hand-off message to the UAV radio sub-systemrequesting that the terminal be handed off to the candidate hand-offbeam. The UAV radio sub-system subsequently associates the terminal tothe candidate hand-off beam and routes all packets from/to the terminalto the candidate hand-off beam. In other variants, the terminal mayidentify multiple candidate hand-off beams, which the UAV radiosub-system selects for hand-off. In still other variants, the terminalmay simply initiate a hand-off to the candidate hand-off beam withoutrequiring UAV instruction.

In another embodiment, the terminal or the UAV radio sub-system maydecide to carry out a hand-off from the UAV beam currently communicatingwith the terminal (so called serving beam) to another UAV beam based onthe UAV position coordinates and orientation that are reported by theUAV to the terminals. The terminal or the UAV radio sub-system can usethe UAV position coordinates and orientation to determine the relativeposition of different UAV beams and decide whether hand-off to anotherbeam is desired according to a certain performance criterion. Forinstance, the terminal or the UAV radio sub-system may estimate thedownlink/uplink SINR seen at the terminal or the UAV using the UAVantenna patterns seen at the terminal corresponding to the UAV positioncoordinates and orientation; based on that information, the terminal mayevaluate the beams that provide coverage to the terminal, and initiate ahand-off if a beam other than the serving beam provides a higher SINR.Artisans of ordinary skill in the related arts may substitute otherhand-off techniques with equivalent success, given the contents of thepresent disclosure. Artisans of ordinary skill in the related arts maysubstitute other hand-off techniques with equivalent success, given thecontents of the present disclosure.

For embodiments where the UAV beams are fixed with respect to the UAVand the UAV beams move on the ground as the UAV travels in its cruisingorbit, handoffs may occur frequently. For example, in some cases theterminal may experience hand-offs from one UAV beam to another as theUAV travels around its orbit. Since, the UAV serving beam and theco-frequency interfering beams for the terminal will change afterhand-off, thus for such embodiments, the table of UAV serving beam gainand the co-frequency interfering beam gains toward each terminal mayinclude information for multiple UAV serving beams so as to bettercompare among candidate beams.

For embodiments which have beams that are fixed on certain locations onthe ground, as illustrated in FIG. 3C, the UAV processor 314 sends theposition location and orientation of the UAV provided bygyroscope/accelerometer/GPS sub-system 319 of the UAV radio sub-system112, to the UAV antenna sub-system 114; the UAV antenna sub-system 114uses the information to steer the beam to be fixed on a given locationto compensate for the UAV movements as the UAV travels in its cruisingorbit. This information is sufficient (along with the terminal's ownlocation information) to predict the likely candidate beams.

When a terminal is inactive (i.e., not sending/receiving data), theterminal may go into a lower power mode (e.g., a sleep mode, an idlemode, and/or other reduction in power consumption.) Whenever theterminal has data to send to the UAV (e.g., based on periodic updates orother triggering event), the terminal wakes up out of sleep mode andaccesses the uplink shared channel to send data and request a dedicateduplink channel (if needed). In one exemplary variant, the terminal willscan for different beams and choose the beam from that has the strongestreceived signal with which to communicate with the UAV. The terminal mayalso periodically wake up during specific time slots so that the UAV caninform the terminal that it has data to transmit to the terminal; thesetime slots are referred to as “wake up time slots” in this disclosure.

In some variants, the terminal is assigned a wake up time slot by e.g.,the UAV or other network management entity. For example, once the UAVhas data to send to a terminal that is in sleep mode, the UAV radiosub-system will wait for the wake up time slot, and then send a wake upmessage to the terminal during the terminal's previously determined wakeup time slot. The wake up message informs the terminal that it needs toprepare to receive data, i.e. come out of sleep mode.

In other variants, the terminal and UAV share a common mechanism fordetermining a wake up time slot (e.g., via a shared hashing algorithm,etc.) For example, the terminal may listen to a wake up time slot of abeam that has the strongest signal. The UAV radio sub-system may,however, not know which beam the terminal is monitoring. Since theterminal does not know which UAV beam and corresponding wake up timeslot the UAV radio sub-system is monitoring, in some cases, the UAVradio sub-system sends the wake up message destined to a given terminalon all beams during the wake up time slot, ensuring the terminal willreceive the wake up message. Such broadcast schemes may consumesignificant network resources. Thus, in another embodiment, the UAVacquires or estimates the terminal's position coordinates (e.g., basedon out-of-band information, historic use information, and/or otheractual/prediction of location information) and based on the currentposition and orientation of the UAV, the UAV radio sub-system sends thewake message on an appropriately selected sub-set of the beams that areproviding coverage to the terminal.

UAV Antenna Design—

FIG. 6A shows an exemplary UAV antenna fixture. As shown, the exemplaryantenna fixture is composed of 7 faces, labeled as 116-j where j is theindex of the different faces j=1, 7. Face 160-1 covers the area underthe aerial platform that is closer to the center of coverage. Thetrapezoidal base of the antenna fixture is attached to underneath theaerial platform. Faces 116-2 through 116-6 cover areas that are fatherfrom the center of coverage of the UAV. Each of the 7 faces of theantenna fixture of FIG. 6A may be considered a different antennaaperture covering a different geographic area. The 7 faces may beseparate and placed on different locations under the UAV to coverdifferent areas on the ground. Artisans of ordinary skill in the relatedart may substitute antenna fixtures with any number of antenna apertures(similar to faces of the antenna fixture of FIG. 6A), given the contentsof the present disclosure.

In some variants, each antenna face 116-j includes multiple antennasub-apertures 117-k, where k is the label of different sub-apertures.Each sub-aperture 117-k generates one beam toward one cell of thecoverage area. Each antenna sub-aperture 116-j is attached to anactuator 119-k which is controlled by processor 314. Once the processor314 has computed the pointing angles of each antenna sub-aperture 116-k,it instructs the actuator 119-k to tilt the sub-aperture 117-k accordingto the computed pointing angle.

In one aspect of the antenna system design, the antenna beam iselectronically formed via e.g., a phased array, to point the beams thatcover each cell on the ground. As shown in FIG. 6B, each antenna facecontains multiple antenna elements 115-j spaced at substantially halfthe transmission and/or reception wavelength, where j is the label ofthe different antenna elements. The same antenna aperture face maygenerate multiple beams, each beam covering a different area on theground. In most practical applications, the transmission and receptionwavelengths are not significantly different (e.g., only differing by afew megahertz, at gigahertz frequency carrier ranges), accordingly thehalf wavelength distance is predominantly based on the carrierfrequency. However, those of ordinary skill in the related arts willreadily appreciate, given the contents of the present disclosure, thatwhere the transmission and reception wavelengths are substantiallyseparated, the half wavelength distance would be different betweentransmission and reception antenna fixtures.

Once the processor 314 has computed the pointing angles for beams towardeach cell on the ground, it instructs the antenna sub-system 114 to formbeams toward each cell at the corresponding pointing angles. FIG. 6Cillustrates a phased array beam forming approach where the antennasub-system 114 forms each beam by multiplying the signal destined forthe k—the beam by coefficients C_(jk) (j=1, . . . , N) and sending theresults to a subset of N antenna elements 115-1 through 115-N.

Terminal Antenna Steering Design—

As discussed above, and depicted in FIG. 3A, the UAV 110 travels arounda cruising orbit 610. The UAV also moves to different altitudes duringthe day and night, higher altitudes during the day and lower altitudesat night. In order to maintain high antenna gain toward the UAV, theterminal antenna 124 steers its beam toward the UAV to track theposition of the UAV. An electronically steered antenna, such as phasedarray technology, is one antenna system option to track the UAV. Onedrawback of a purely electronically steerable antenna approaches, suchas the phased array, is their current high cost. For an electronicallysteerable antenna to steer its beam to track the UAV position, theantenna aperture needs to be initially pointed toward the UAV's cruisingorbit. If the terminal antenna is pointed manually toward the UVA, itwill increase the installation cost as effort must be expended tomanually point the antenna during the installation process. Anotherissue with a solely electronically steerable antenna is flexibility. Forexample, if the UAV's position is changed or if additional UAVs aredeployed, and it is desired to point the terminal's antenna toward theUAV's new position or toward the newly deployed UAV, then the terminal'santenna must be manually pointed to the new UAV position. It can be verycostly and time consuming to reposition the antennas of a large numberof installed terminals.

A mechanically steerable antenna has the benefit that during theinstallation the terminal may mechanically steer its antenna toward theUAV cruising orbit without any manual intervention and the associatedcost. The terminal antenna sub-system 124 depicted in FIG. 7, includestwo axis mechanical steering mechanisms 127 and 128. At installationtime, the terminal radio sub-system 122 mechanically steers the antennaaperture 126 to different positions and searches for the signal 212(FIG. 1) until it detects the signal transmitted by the UAV 110. Oncethe signal from the UAV is detected, the terminal radio sub-system 122may continue the search process to more finely point the antennaaperture 126 toward the UAV 110.

If the beamwidth of the antenna is very narrow, say less than four (4)degrees, then one potential issue with mechanically steerable antenna isits ability to point a beam accurately toward the UAV while maintainingthe cost low antenna cost for home terminals. Generally, it is possibleto design a good enough mechanically steerable structure for accuratebeam pointing, but the cost of the mechanical parts (e.g., at thatmanufacturing tolerance) may be too high for a consumer product.Therefore, a mechanism is needed to reduce the beam pointing requirementof the mechanical structure while achieving high antenna gain and lowterminal cost. In one embodiment of this disclosure, multiple antennasub-apertures 118-j, j index of the apertures of the same type whosebeamwidth is larger than that of aperture 126, are used to create thelarger antenna aperture 126. For instance, one can create an antennawith same aperture size by combining four (4) smaller antennasub-apertures, each having an area one quarter that of the largerantenna aperture; by combining signals from the four (4) smaller antennasub-apertures, six (6) decibels (dB) can be gained which results in anequivalent net gain to that of the larger single aperture. However, thebeamwidth of each of the four (4) smaller sub-apertures will be halfthat of the larger antenna. As a result, the mechanical beam pointingaccuracy requirement is reduced due to the wider beamwidth of each ofthe four (4) smaller sub-apertures.

In a phased array approach, antenna elements are placed a halfwavelength apart and a large number of antenna elements are needed toachieve high gain. For instance, a circularly shaped antenna array at 28GHz of diameter 30 cm requires about 666 antenna elements to achieve ahigh antenna directivity of about 36 dB. The beamwidth of such anantenna would be about 2.5 degrees. In the antenna design described inthis disclosure, typically four (4) to eight (8) antenna sub-aperturesare used to create a larger antenna aperture as described above. If four(4) dish antennas with diameter of 15 cm each, and 65% efficiency, areused as sub-apertures to create the larger antenna, then the beamwidthof each of the four (4) antenna will be about five (5) degrees versusthe 2.5 degree beamwidth of the larger 30 cm diameter antenna. The gainof the combined four (4) 15 cm dishes is the same as that of the larger30 cm antenna dish. If eight (8) dishes with size about 10 cm are used,then each of the 10 cm dishes will have a beamwidth of about seven (7)degrees and the combined gain of the 10 cm antenna dishes will be thesame as that of the single 30 cm dish. Therefore, by using eight (8)dishes to create one larger dish the antenna assembly can provide a gainof the larger dish while significantly reducing the beam pointingaccuracy requirement for the mechanical steering scheme. The complexityand cost of an antenna that includes four (4) to eight (8) antennasub-apertures and combines the signals digitally (or via analog mixing)is much smaller than that of a phased array with about 666 antennaelements in the above example.

In one exemplary embodiment of the present disclosure, the terminalantenna structure of FIG. 7 is a hybrid of mechanical steering andelectronic beam forming using multiple antenna sub-apertures. Thisdesign combines the benefit of mechanical and electronic steering toreduce the requirements on the mechanical steering mechanism whilemaintaining low complexity for the electronic beam forming mechanism.The hybrid scheme maintains accurate beam pointing to maximize the gaintoward the UAV while reducing the complexity of antenna design andachieving low cost antenna constructions.

Referring now to FIG. 7, the antenna sub-apertures 118-j are larger thanthe size of antenna elements needed in phased array antennas (such asdepicted in e.g., FIGS. 5A-5C). In other words, sub-apertures 118-j arethemselves antennas such as dish antennas or flat panel antennas withhigh gains. Each of the antenna sub-apertures 118-j are connected to thetransmitter 416 and receiver 418 sub-systems of the radio sub-system 122via antenna connections 417-j and 419-j respectively as shown in FIG. 7.In order to achieve higher gain than that of each sub-aperture 118-j,signals received from multiple antenna sub-apertures 118-j may becombined digitally in baseband or in the analog domain by applyingappropriate phases to the signal from each antenna aperture. On thetransmit side, the signal may be transmitted only through one of thesub-apertures 118-j but at higher PA (Power Amplifier) transmit power inorder to compensate for the lower antenna gain of a single sub-aperture118 versus the gain of the larger aperture 126. Alternatively, thetransmit signal may be sent to all antenna sub-apertures 188-j, wherethe signal is phased appropriately and sent to each of the sub-apertures118-j to form a narrower, and higher gain beam toward the UAV than thatof one sub-aperture 118-j.

In one embodiment, the terminal radio sub-system 122 uses the UAVposition coordinates to point the antenna 126 toward the UAV using themechanical steering mechanism. The position coordinates of the UAV areperiodically sent to all terminals on a downlink message. The antennadesign of this disclosure, depicted in FIG. 7, is capable of mechanicalsteering as well as electronic steering using the antenna sub-apertures118-j. The mechanical antenna steering mechanism may have an error dueto effects such as the stepper motor backlash or backlash due to windloading. During each time slot, the receiver sub-system 418 combines thesignals R_(k) received on each antenna sub-aperture connection 419-kusing gains α_(k) and phases ϕ_(k) applied to the signals R_(k)according to equationΣ_(k) R _(k)α_(k) e ^(jϕ) ^(k)   (8)The antenna sub-aperture signal combining equation (8) may be optimallycombined in baseband or in analog domain to maximize a signal qualitymetric such as SINR.

Similarly, on the transmit side, the gain and phase of signal S_(k) tobe transmitted may be adjusted for each antenna sub-aperture 118-k sothat the resulting transmit beam “optimally” points toward the UAV. Thegain and phase applied on the receive side, shown in equation (8), mayalso be applied to the signal being transmitted on each transmit antennawhile correcting for the calibration difference between transmit andreceive chains. The transmit signals sent to antenna sub-aperture 118-kis then given by S_(k) α_(k)e^(jϕ) ^(k) ⊕_(k)e^(jϕ) ^(k) , where β_(k)and θ_(k) are the gain and phase adjustment needed for each transmitantenna sub-aperture to account for the calibration difference betweenthe transmit and receive chains, as well as the frequency differencebetween the uplink and downlink in FDD (Frequency Division Duplex)systems. In other words, β_(k) and θ_(k) account for the hardware delay(phase) and gain differences in receive and transmit directions.

The terminal antenna steering mechanism described above is configured totrack the movements of the UAV using mechanical steering as well ascombining signals from multiple antenna apertures; this terminal antennasteering mechanism may also be used to track LEO (Low Earth Orbit)satellites. One exemplary antenna design is based on a phase arrayantenna that can electronically steer a beam to track a satellite. Insome variants, a mechanical beam steering mechanism may also be neededto track the movement of the satellites; mechanical steering may beparticularly important where the field of view of the phased arrayantenna is likely to be smaller than the spacing between two adjacentsatellites. However, mechanical beam steering can add to componentcosts; for example, as described above in connection with tracking theUAV movement, the cost considerations for the mechanical steeringmechanism may be affected by use scenarios. For example, where thebeamwidth of the antenna is narrow, the mechanical mechanism may be moreexpensive (e.g., tighter tolerances are required where the beamwidth ofthe antenna is very narrow). Moreover, the mechanical steeringperformance may be susceptible to wind loading or other weatherconditions (which does not affect electrical steering). Therefore, thehybrid antenna design illustrated in FIG. 7 and described in previousembodiments may achieve accurate antenna beam pointing toward thesatellite while achieving low cost for the antenna system.

It will be recognized that while certain embodiments of the presentdisclosure are described in terms of a specific sequence of steps of amethod, these descriptions are only illustrative of the broader methodsdescribed herein, and may be modified as required by the particularapplication. Certain steps may be rendered unnecessary or optional undercertain circumstances. Additionally, certain steps or functionality maybe added to the disclosed embodiments, or the order of performance oftwo or more steps permuted. All such variations are considered to beencompassed within the disclosure and claimed herein.

While the above detailed description has shown, described, and pointedout novel features as applied to various embodiments, it will beunderstood that various omissions, substitutions, and changes in theform and details of the device or process illustrated may be made bythose skilled in the art without departing from principles describedherein. The foregoing description is of the best mode presentlycontemplated. This description is in no way meant to be limiting, butrather should be taken as illustrative of the general principlesdescribed herein. The scope of the disclosure should be determined withreference to the claims. It will be further appreciated that whilecertain steps and aspects of the various methods and apparatus describedherein may be performed by a human being, the disclosed aspects andindividual methods and apparatus are generallycomputerized/computer-implemented. Computerized apparatus and methodsare necessary to fully implement these aspects for any number of reasonsincluding, without limitation, commercial viability, practicality, andeven feasibility (i.e., certain steps/processes simply cannot beperformed by a human being in any viable fashion).

What is claimed is:
 1. An unmanned aerial vehicle (UAV) broadband access system comprising: an antenna sub-system comprising at least one antenna aperture configured to form a plurality of beams toward a ground coverage area, where the plurality of beams are further subdivided into a plurality of groups of co-active beams; wherein at least a portion of the plurality of groups of co-active beams are co-frequency; a UAV radio sub-system comprising one or more transmitters and receivers and configured to transmit and receive signals to and from a set of ground terminals within the ground coverage area; and wherein the UAV radio sub-system further comprises logic configured to: assign at least one ground terminal in at least one beam to transmit during a given time slot; estimate one or more UAV downlink beam gains for the at least one beam assigned to the at least one ground terminal based on one or more of: one or more UAV position coordinates, one or more UAV orientations, and at least one ground terminal location; determine an optimal downlink data rate for the at least one beam based at least in part of the estimated one or more UAV downlink beam gains; and transmit downlink data packets to the assigned at least one ground terminal in the at least one beam based at least in part on the determined optimal downlink data rate.
 2. The UAV broadband access system of claim 1, wherein the UAV radio sub-system further comprises logic configured to: determine an optimal uplink data rate that can be decoded on an uplink based on the measured uplink signal strength and at least one criteria; and receive uplink data packets based at least in part on the determined optimal uplink data rate.
 3. The UAV broadband access system of claim 2, wherein the UAV radio sub-system further comprises logic configured to send the measured uplink signal strength or the determined optimal uplink data rate to the assigned at least one ground terminal.
 4. The UAV broadband access system of claim 1, wherein the UAV radio sub-system further comprises logic configured to: schedule downlink transmissions to the assigned at least one ground terminal in the at least one beam; and receive measured downlink signal strength estimates from the assigned at least one ground terminal on the at least one co-active co-frequency beam.
 5. The UAV broadband access system of claim 4, wherein the UAV radio sub-system further comprises logic configured to: determine an optimal downlink data rate based at least in part on the received measured downlink signal quality estimates and at least one other criteria; and transmit downlink data packets based at least in part on the determined optimal downlink data rate.
 6. The UAV broadband access system of claim 1, wherein the UAV radio sub-system further comprises logic configured to: estimate one or more UAV uplink beam gains based at least in part on one or more of: one or more UAV position coordinates, one or more UAV orientations, and at least one ground terminal location; determine an optimal uplink data rate based at least in part of the estimated one or more UAV uplink beam gains; and receive uplink data packets based at least in part on the determined optimal uplink data rate.
 7. The UAV broadband access system of claim 6, wherein the estimated one or more UAV uplink beam gains are performed at a plurality of UAV positions on a cruising orbit.
 8. The UAV broadband access system of claim 1, wherein the UAV radio sub-system further comprises logic configured to: compute an uplink interference power of an interfering uplink of at least one other ground terminal that transmits during a same time slot as the assigned at least one terminal; determine an optimal uplink data rate based at least in part on the computed uplink interference power; and receive uplink data packets based at least in part on the determined optimal uplink data rate.
 9. The UAV broadband access system of claim 1, wherein the UAV radio sub-system further comprises logic configured to: compute a total uplink interference based at least in part on all ground terminals that are scheduled to transmit at a same time slot as the assigned at least one ground terminal; compute an expected signal strength from the assigned at least one ground terminal; compute an expected uplink signal quality of the assigned at least one ground terminal based on the computed total uplink interference and the computed expected signal strength from the assigned at least one ground terminal; determine an optimal uplink data rate based at least in part of the computed expected uplink signal quality at the assigned at least one ground terminal; and receive uplink data packets based at least in part on the determined optimal uplink data rate.
 10. The UAV broadband access system of claim 1, wherein the UAV radio sub-system further comprises logic configured to: compute an downlink interference power of an interfering downlink of at least one other ground terminal that receives during a same time slot as the assigned at least one ground terminal; determine an optimal downlink data rate based at least in part on the computed downlink interference power; and transmit downlink data based at least in part on the determined optimal downlink data rate.
 11. The UAV broadband access system of claim 1, wherein the UAV radio sub-system further comprises logic configured to: compute a total downlink interference based on all ground terminals that are scheduled to receive at a same time slot as the assigned at least one ground terminal; compute an expected signal strength at the assigned at least one ground terminal; compute an expected downlink signal quality of the assigned at least one ground terminal based on the computed total downlink interference and the computed expected signal strength at the assigned at least one ground terminal; determine an optimal downlink data rate based at least in part of the computed expected downlink signal quality at the assigned at least one ground terminal; and transmit downlink data packets based at least in part on the determined optimal downlink data rate.
 12. A method for receiving data packets in a UAV broadband access system, comprising: forming at least one beam toward a ground coverage area; measuring an uplink signal estimate of received signals on the formed at least one beam; determining an optimal uplink data rate that can be decoded on the uplink based on the measured uplink signal estimate and at least one criteria; and receiving uplink data packets on the formed at least one beam based at least in part on the determined optimal uplink data rate.
 13. The method of claim 12, further comprising: transmitting a reference signal on at least one other beam; receiving a measured downlink signal estimate of the at least one other beam from at least one ground terminal within the ground coverage area; determining an optimal downlink data rate based at least in part on the received measured downlink signal estimate and at least one other criteria; and transmitting downlink data packets based at least in part on the determined optimal downlink data rate.
 14. The method of claim 13, further comprising transmitting the measured uplink signal estimate to the at least one ground terminal within the ground coverage area.
 15. A ground terminal apparatus configured to communicate with an unmanned aerial vehicle (UAV) broadband access system, the ground terminal apparatus comprising: a radio transceiver configured to transmit and receive signals to and from the UAV; a processor coupled to the radio transceiver; and logic configured to: determine one or more beams that are associated with the UAV; determine one or more time slots that are assigned for transmission; transmit one or more reference signals according to the determined one or more time slots and determined one or more beams; measure a downlink signal estimate of received signals on a downlink; determine an optimal downlink data rate that can be decoded on the downlink based on the measured downlink signal estimate and at least one criteria; receive downlink data packets based at least in part on the determined optimal downlink data rate; and wherein the transmitted one or more reference signals are associated with the UAV.
 16. The ground terminal apparatus of claim 15, wherein each group of co-active beams are further associated with one or more frequencies.
 17. The ground terminal apparatus of claim 15, wherein the one or more reference signals are further encoded with a UAV beam specific code that identifies the determined one or more beams from a plurality of beams.
 18. The ground terminal apparatus of claim 15, wherein the determined one or more beams are further associated with a group of co-active beams. 